Because we try as much as possible to let the compiler figure this sort of
thing out for us, we implement the transparent lookups for this & self by
compiling our expression in a context that poses as a method of the class whose
method we are stopped in. For instance, for ObjC, we construct a category on
the class, and put the expression text in a method in that category, and then
compile all that and call the method.
That's done in ExpressionSourceCode::GetText.
The one hitch to this is that this works because the compiler that is linked
into lldb knows about the ObjC model we are emulating. An unmodified clang
won't do transparent lookup into some random argument that happens to be called
_param. So you would have to build lldb with a version of clang that
understands your lookup rules for this to work.
If that's not possible then you can try to do this by monkeying with the lookup
rules implemented by lldb's ClangASTSource to return "_param.a" when it is just
looking up "a". That's how we inject variables and types that are present in
the local context into the expression as it is getting parsed. But at that
point you are getting your hands into some fairly deep magic, and clang's Aslan
is not as forgiving as Narnia's...
Jim
> On May 26, 2017, at 2:27 PM, Nat! via lldb-dev <lldb-dev@lists.llvm.org>
> wrote:
>
> Let me show you a snippet of a lldb debug session in progress in my ObjC
> variant:
>
> ```
> -100000,100000,v,18.48
> Process 45774 stopped
> * thread #1, queue = 'com.apple.main-thread', stop reason = step in
> frame #0: 0x0000000100000e2a multiple.debug`+[Foo
> long:int:char:float:](self=Foo, _cmd=<no value available>,
> _param=0x00007fff5fbff948) at multiple.m:15
> 12 char:(char) c
> 13 float:(float) d
> 14 {
> 15 printf( "%ld,%d,%c,%.2f\n", a, b, c, d);
> -> 16 }
> 17
> 18 @end
> (lldb) p *_param
> (p.long:int:char:float:) $2 = (a = -100000, b = 100000, c = 'v', d =
> 18.4799995)
> ```
>
> You can see that the parameter values `a,b,c,d` are actually fields of
> a struct parameter `_param`. `_param` uniformly appears as the third
> parameter after `self` and `_cmd`. `p _param->a` works of course, but it
> would be nice to be able to say 'p a', since in the source code one sees
> only `a`. `_param` is more or less an implementation detail.
>
> A clue how to achieve this, would be very much appreciated.
>
> Ciao
> Nat!
>
>
> [*] except, if it's a picture of thousand words :)
>
> https://www.mulle-kybernetik.com/weblog/2015/mulle_objc_meta_call_convention.html
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